1. Field of the Invention
This invention relates to a radiation image read-out method and a radiation image read-out apparatus, and more particularly to a radiation image read-out method and a radiation image read-out apparatus for reading out a radiation image stored on a stimulable phosphor sheet by the use of a line sensor.
2. Description of the Related Art
When certain kinds of phosphor are exposed to a radiation such as X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet rays and the like, they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as “a stimulable phosphor”. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as “stimulated emission”. There has been known a radiation image recording and reproducing system in which a stimulating light beam such as a laser beam is projected onto a stimulable phosphor sheet (a sheet provided with a layer of the stimulable phosphor) which has been exposed to a radiation passing through an object such as a human body to have a radiation image of the object stored on the stimulable phosphor sheet, and the stimulated emission emitted from the stimulable phosphor sheet is photoelectrically detected, thereby obtaining an image signal (a radiation image signal) representing a radiation image of the object. See, for instance, Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395 and 56(1981)-11397.
As the apparatus used for reading out the radiation image signal, there have been known those of various structures. For example, there has been known an apparatus in which a stimulable phosphor sheet contained in a cassette is taken out from the cassette and is placed on a conveyor belt, stimulating light is projected onto the stimulable phosphor sheet while it is being conveyed by the conveyor belt and stimulated emission emitted from each part of the stimulable phosphor sheet upon exposure to the stimulating light is detected.
As a system for detecting stimulated emission emitted from the stimulable phosphor sheet, there have been known a system in which stimulated emission imaged on a line sensor by an imaging optical system is detected by the line sensor, and a system in which stimulated emission is detected by a line sensor held in the vicinity of the stimulable phosphor sheet, thereby obtaining information on the position of the stimulable phosphor sheet from which the stimulated emission is emitted and information on the intensity of the stimulated emission and the reading out a radiation image recorded on the stimulable phosphor sheet.
In the latter system, a line sensor having a plurality of photoelectric convertor elements arranged in a row is held in the vicinity of or in contact with the stimulable phosphor sheet and is caused to directly detects the stimulated emission emitted from the stimulable phosphor sheet. In the former system, the imaging optical system may be any optical system so long as it can form on the line sensor an image of the surface of the stimulable phosphor sheet from which the stimulated emission is emitted. For example, the imaging optical system need not be an optical system in which an image of the surface of the stimulable phosphor sheet is formed on the line sensor by the action of the lens but may be those in which an image of the surface of the stimulable phosphor sheet is formed on the line sensor by leading the stimulated emission to the line sensor by way of image fibers.
In either system, a radiation image is read out from the stimulable phosphor sheet by leading the stimulated emission emitted from a particular area on the stimulable phosphor sheet to a corresponding photoelectric convertor element of the line sensor.
However, when a radiation image recorded on a stimulable phosphor sheet is to be read out by the use of a line sensor, the distance between the stimulable phosphor sheet and the imaging optical system or the distance between the stimulable phosphor sheet and the line sensor greatly affects the quality of the radiation image.
For example, in the case where an image of the area of the stimulable phosphor sheet emitting the stimulated emission is formed on the line sensor by an imaging optical system, the image can come out of focus and the image signal representing a radiation image read out from the stimulable phosphor sheet can be deteriorated in S/N when the distance between the stimulable phosphor sheet and the imaging optical system (will be referred to as “the detecting space”, hereinbelow.) fluctuates. Similarly, in the case where the stimulated emission emitted from the stimulable phosphor sheet is directly detected by the line sensor without an imaging optical system, fluctuation in the detecting space (i.e., the distance between the stimulable phosphor sheet and the line sensor in this case) changes the amount of stimulated emission which enters each photoelectric convertor element of the line sensor from the corresponding area of the stimulable phosphor sheet and the proportion of the amount of stimulated emission which enters each photoelectric convertor element of the line sensor from the area of the stimulable phosphor sheet not corresponding to the photoelectric convertor element to that of stimulated emission which enters each photoelectric convertor element of the line sensor from the corresponding area of the stimulable phosphor sheet. That is, as the detecting space becomes smaller, the amount of stimulated emission which enters each photoelectric convertor element of the line sensor from the corresponding area of the stimulable phosphor sheet increases and the proportion of the amount of stimulated emission which enters each photoelectric convertor element of the line sensor from the area of the stimulable phosphor sheet not corresponding to the photoelectric convertor element to that of stimulated emission which enters each photoelectric convertor element of the line sensor from the corresponding area of the stimulable phosphor sheet increases, whereby the image signal read out becomes higher in S/N, whereas as the detecting space becomes larger, the amount of stimulated emission which enters each photoelectric convertor element of the line sensor from the corresponding area of the stimulable phosphor sheet decreases and the proportion of the amount of stimulated emission which enters each photoelectric convertor element of the line sensor from the area of the stimulable phosphor sheet not corresponding to the photoelectric convertor element to that of stimulated emission which enters each photoelectric convertor element of the line sensor from the corresponding area of the stimulable phosphor sheet decreases, whereby the image signal read out becomes lower in S/N, which can result in fluctuation of image quality from area to area due to fluctuation in the amount of stimulated emission received by each photoelectric convertor element and/or the S/N.
However, since the detecting space has been conventionally fluctuated due to distortion of the stimulable phosphor sheet and/or errors in conveying the stimulable phosphor sheet, there has been a demand for suppressing fluctuation in the detecting space as small as possible.